Once they are more than a month old, babies seem to learn readily. Moreover, solving a pro

blem or discovering the relationship between their own actions and an event in the external world seems to have its own rewards for infants.

The pleasure involved in such an experience seemed obvious in one study of two-to-four- month-old infants who were given a chance to control a yellow-and-green mobile. One of these mobiles was suspended above each infant, who play in a crib (小床) with a ribbon attached to one ankle. The other end of the ribbon was attached to the hook from which the mobile was suspended. As the infants moved about, waving their arms and kicking their legs, they learned to connect the motion of one leg with the bobbing(跳动) of the mobile. As they smiled and gurgled(咯咯的笑) at the moving mobile, the infants began to kick the leg attached to the mobile forcefully and precisely and only that leg. Apparently, they enjoyed controlling the mobile, for they would continue for as long as 45 minutes.

Nor is this pleasure in mastery limited to infants. In an experiment with fifth and sixth graders, the children were asked to solve some word puzzles that varied in difficulty. The youngsters smiled more, and reported far more pleasure, when they solved a difficult puzzle than any easy one. The implication is that human beings of any age-from infancy to old age- derive pleasure from intellectual mastery.

In the experiment, the infants ______.

A．kicked their legs randomly all the time

B．kicked the leg controlling the mobile most of the time

C．attached the colorful ribbons to their own legs

D．felt very tired and lost their interest soon